Protein kinases are a family of enzymes that catalyze the phosphorylation of proteins, in particular the hydroxy group of specific tyrosine, serine and threonine residues in proteins. Protein kinases play a critical role in the regulation of a wide variety of cellular processes, including metabolism, cell proliferation, cell differentiation, cell survival, environment-host reaction, immune response, and angiogenesis. Many diseases are associated with abnormal cellular responses triggered by protein kinase-mediated events. These diseases include inflammatory diseases, autoimmune diseases, cancer, neurological and neurodegenerative diseases, cardiovascular diseases, allergies and asthma or hormone-related disease (Tan, S-L., 2006, J. Immunol., 176: 2872-2879; Healy, A. ea al., 2006, J. Immunol., 177: 1886-1893; Salek-Ardakani, S. et al., 2005, J. Immunol., 175: 7635-7641; Kim, J. et al., 2004, J. Clin. Invest., 114: 823-827). Therefore, considerable effort has been made to identify protein kinase inhibitors that are effective as therapeutic agents against these diseases.
The protein kinases can be conventionally divided into two classes, the protein tyrosine kinases (PTKs) and the serine-threonine kinases (STKs).
The protein tyrosine kinases (PTKs) are divided into two classes: the non-transmembrane tyrosine kinases and transmembrane growth factor receptor tyrosine kinases (RTKs). At present, at least nineteen distinct subfamilies of RTKs have been identified, such as the epidermal growth factor receptor (EGFR), the vascular endothelial growth factor receptor (VEGFR), the platelet derived growth factor receptor growth factor receptor (PDGFR), and the fibroblast growth factor receptor (FGFR).
The epidermal growth factor receptor (EGFR) family comprises four transmembrane tyrosine kinase growth factor receptors: HER1, HER2, HER3 and HER4. Binding of a specific set of ligands to the receptor promotes EGFR dimerization and results in the receptors autophosphorylation on tyrosine residues (Arteaga, C-L., 2001, Curr. Opin. Oncol., 6: 491-498). Upon autophosphorylation of the receptor several signal transduction pathways downstream of EGFR become activated. The EGFR signal transduction pathways have been implicated in the regulation of various neoplastic processes, including cell cycle progression, inhibition of apoptosis, tumor cell motility, invasion and metastasis. EGFR activation also stimulates vascular endothelial growth factor (VEGF), which is the primary inducer of angiogenesis (Petit, A-M. et al., 1997, Am. J. Pathol., 151: 1523-1530). In experimental models, deregulation of the EGFR-mediated signal transduction pathways is associated with oncogenesis (Wikstrand, C-J. et al., 1998, J Natl Cancer Inst., 90: 799-800). Mutations leading to continuous activation of amplification and over expression of EGFR proteins are seen in many human tumors, including tumors of breast, lung, ovaries and kidney. These mutations are a determinant of tumor aggressiveness (Wikstrand, C-J. et al., 1998, J Natl Cancer Inst., 90: 799-800). EGFR over expression is frequently seen in non-small cell lung cancer (NSCLC). Activity of EGFR can be inhibited either by blocking the extracellular ligand binding domain with the use of anti-EGFR antibodies or by the use of small molecules that inhibit the EGFR tyrosine kinase, thus resulting in inhibition of downstream components of the EGFR pathway (Mendelsohn, J., 1997, Clin. Can. Res., 3: 2707-2707).
The vascular endothelial growth factor (VEGF) is secreted by almost all solid tumors and tumor associated stroma in response to hypoxia. It is highly specific for vascular endothelium and regulates both vascular proliferation and permeability. Excessive expression of VEGF levels correlate with increased microvascular density, cancer recurrence and decreased survival (Parikh, A-A., 2004; Hematol. Oncol. Clin. N. Am., 18:951-971). There are 6 different ligands for the VEGF receptor, VEGF-A through-E and placenta growth factor. Ligands bind to specific receptors on endothelial cells, mostly VEGFR-2. The binding of VEGF-A to VEGFR-1 induces endothelial cell migration. Binding to VEGFR-2 induces endothelial cell proliferation, permeability and survival. VEGFR-3 is thought to mediate lymphangiogenesis. The binding of VEGF to VEGFR-2 receptors results in activation and autophosphorylation of intracellular tyrosine kinase domains which further triggers other intracellular signaling cascades (Parikh, A-A., 2004, Hematol. Oncol. Clin. N. Am., 18:951-971).
The serine-threonine kinases (STKs) are predominantly intracellular although there are a few receptor kinases of the STK type. STKs are the most common forms of the cytosolic kinases that perform their function in the part of the cytoplasm other than the cytoplasmic organelles and cytoskelton.
Glycogen synthase kinase-3 (GSK-3) is a serine-threonine protein kinase comprised of α and β isoforms that are each encoded by distinct genes. GSK-3 has been found to phosphorylate and modulate the activity of a number of regulatory proteins. GSK-3 has been implicated in various diseases including diabetes, Alzheimer's disease, CNS disorders such as manic depressive disorder and neurodegenerative diseases, and cardiomyocyte hypertrophy (Haq, et al., 2000, J. Cell Biol., 151: 117).
Aurora-2 is a serine-threonine protein kinase that has been implicated in human cancer, such as colon, breast, and other solid tumors. This kinase is believed to be involved in protein phosphorylation events that regulate cell cycle. Specifically, Aurora-2 may play a role in controlling the accurate segregation of chromosomes during mitosis. Misregulation of the cell cycle can lead to cellular proliferation and other abnormalities. In human colon cancer tissue, the Aurora-2 protein has been found to be over expressed (Schumacher, et al., 1998, J. Cell Biol., 143: 1635-1646; Kimura et al., 1997, J. Biol. Chem., 272: 13766-13771).
The cyclin-dependent kinases (CDKs) are serine-threonine protein kinase that regulates mammalian cell division. CDKs play a key role in regulating cell machinery. To date, nine kinase subunits (CDK 1-9) have been identified. Each kinase associates with a specific regulatory partner and together makes up the active catalytic moiety. Uncontrolled proliferation is a hallmark of cancer cells, and misregulation of CDK function occurs with high frequency in many important solid tumors. CDK2 and CDK4 are of particular interest because their activities are frequently misregulated in a wide variety of human cancers.
Raf kinase, a downstream effector of ras oncoprotein, is a key mediator of signal-transduction pathways from cell surface to the cell nucleus. Inhibition of raf kinase has been correlated in vitro and in vivo with inhibition of the growth of variety of human tumor types (Monia et al., 1996, Nat. Med., 2: 668-675).
Other serine-threonine protein kinases include the protein kinase A, B and C. These kinases, known as PKA, PKB and PKC, play key roles in signal transduction pathways.
Many attempts have been made to identify small molecules which act as protein kinase inhibitors useful in the treatment of diseases associated with abnormal protein kinase activities. For example, cyclic compounds (U.S. Pat. No. 7,151,096), bicyclic compounds (U.S. Pat. No. 7,189,721), tricyclic compounds (U.S. Pat. No. 7,132,533), (2-oxindol-3-ylidenyl)acetic acid derivatives (U.S. Pat. No. 7,214,700), 3-(4-amidopyrrol-2-ylmethlidene)-2-indolinone derivatives (U.S. Pat. No. 7,179,910), fused pyrazole derivatives (U.S. Pat. No. 7,166,597), aminofurazan compounds (U.S. Pat. No. 7,157,476), pyrrole substituted 2-indolinone compounds (U.S. Pat. No. 7,125,905), triazole compounds (U.S. Pat. No. 7,115,739), pyrazolylamine substituted quinazoline compounds (U.S. Pat. No. 7,098,330) and indazole compounds (U.S. Pat. No. 7,041,687) have all been described as protein kinase inhibitors. Several protein kinase inhibitors such as Glivec, Suten, and Sorafenib have been successfully approved by FDA as anti-cancer therapy. Their clinic uses demonstrated clear advantages over existing chemotherapeutical treatments, fueling continuing interests in innovation of mechanism-based treatments and improvement of chemical scaffolds to discover new compounds with excellent oral bioavailability, significant anti-tumor activity, and lower toxicity at well-tolerated dose.